1. Field of the Invention
This invention relates to a rotatable structure containing linear diffraction gratings and in particular to the use of such a structure (such as a disk) to optically decode angular information in a mechanical system.
2. Prior Art
Rotatable holographic disks have been used for optical printers and for optically decoding the angular position of a rotating shaft in a mechanical system. However, they have not been used in a form where their centering is non-critical. Code disks to encode information wherein a number of annular bands possessing alternating opaque and transparent portions are circumferentially mounted at different radii along the disk are known in the art. In these disks, the lengths of the opaque regions dispersed with transparent regions along an annular band are used to determine the particular code to be transmitted by the disk. A light beam impinges the disk, the disk is rotated and the resulting interrupted pattern of light produces a desired code. These disks must be accurately centered and must remain centered to give high quality output signals. They are also subject to dust contamination and therefor must be kept clean.